This invention relates to earth disturbance intrusion detection systems and more particularly to a buried detection system which senses compression waves in the earth produced by human footsteps.
Sensors for measuring and/or detecting vibrations in the earth, often called seismic sensors, generally employ one or more geophones as the transducer element. The geophone, which generally comprises a magnetized core in a helical coil, measures velocity or the rate of change of position of the core relative to the coil in response to earth movements. The earth wave thus is translated first into a mechanical velocity component (movement of core relative to coil) and secondly, into an electrical signal induced by the coil cutting the lines of flux. Such dual transducer action limits sensitivity of the geophone and its capability of measuring vibrations of minimum amplitude and frequency.
In order to enable earth disturbance sensors to be employed as intrusion detectors, they must be able to distinguish vibrations generated by true alarm sources, i.e., a man's footsteps, from false alarm sources, such as aircraft or earthquakes. In fact, the false alarm rating of an intrusion detection system is one of the most important factors involved in judging the effectiveness of the system. One false alarm discrimination technique used by the prior art is the deployment of two separate lines of sensors laterally and equally spaced apart so that broadly generated disturbances such as earthquakes affect both sensors alike whereas localized disturbances typified by an attempted point intrusion affect one sensor differently than the other. By comparing the signals from both sensors in appropriate processing circuits, false alarms are minimized. The disadvantages of such a system are the difficulty and cost of installing duplicate lines of sensors in the ground; these disadvantages are magnified when perimeters measured in miles or even thousands of feet are to be protected.
An object of this invention is the provision of a reliable perimeter type seismic intrusion detection system having a single line of sensors capable of being manufactured and installed at relatively low cost.
Another object is the provision of a seismic sensor having an extremely broad dynamic range, i.e., from less than 1 Hz to greater than 100 Hz.
Still another object is the provision of a seismic sensor that is extremely sensitive, i.e., capable of providing typical seismic compressional wave sensitivities of -90 dB referenced to 1 volt/.mu.bar.
A further object is the provision of a seismic sensor having high detection performance and capable of sensing both seismic and pressure signals generated by the footsteps of a human traversing the sensor.
Still another object is the provision of a seismic sensor which is manufactured from standard commercially available parts which are relatively inexpensive, such as conventional garden hose.
A further object is the provision of a single line seismic detection system having a relatively small diameter and capable of rapid subterranean deployment by standard cable-laying equipment.